Secondary recovery methods to produce oil from subterranean formations include injection of steam, carbon dioxide, and natural gases to drive oil toward recovery wells. Such methods can significantly increase the recovery of oil from many formations, but each is relatively expensive. It is common to use the fuel equivalent of about one-third of the produced oil to produce steam for a steam drive recovery operation. Carbon dioxide and natural gas are also expensive to procure and compress. These recovery processes are therefore economical only if they are carried out in an efficient manner. An important aspect of the efficiency of these processes is the profile of the injection of the drive fluid through multiple sets of well casing perforations and into the formation. This injection profile indicates to the operator, for example, whether portions of the formation are being bypassed, or whether perforations are plugged.
The profile of the injection of drive fluids is typically determined from measuring the velocity of the fluids going down the well borehole, or the velocity profile. This velocity profile is typically determined by either a turbine-type meter or by radioactive tracers. Turbine-type meters are typically lowered into the wellbore on a line. An impeller rotates at a speed which is proportional to the vapor flow, driving a small generator which produces an electrical signal which is proportional to the speed of the impeller rotation. Because turbine meters do not directly measure velocity, but measure the rotation speed of an impeller, they are subject to errors due to calibration. They also require generation of the electronic signal, which relates to the fluid velocity, and transmission of that signal up the borehole.
Radioactive tracers measure velocity by calculation of the time required for a slug of a radioactive tracer to reach detectors which are placed at intervals within the wellbore. The high speeds at which fluids travel down an injection well result in this method rendering data which is of limited resolution. The resolution of the data is particularly troublesome if the sets of perforations are relatively close to each other.
It is also desirable to minimize handling of radioactive tracers. These tracers must not only be handled when injected, but their diluted existence in production fluids must also be considered along with disposal of unused radioactive tracer material.
It is therefore an object of the present invention to provide a method and an apparatus capable of measuring the velocity profile in an injection well wherein radioactive tracers are not required and wherein the velocity can be sampled across a significant portion of the cross-section of the wellbore. It is another object to provide such a method and apparatus which does not require electronic devices or conduction of electronic signals within the wellbore.